Thank you for posting your question here in the Community forum, @Caiged Bookkeeper. 

Right now, QuickBooks doesn't have an option for keeping the HTML format, adding some colour, and company font to the signature. You may consider checking with a third-party application that offers this customization. 

I can see how adding the option to edit the format of your invoices when emailed could help your business exemplify its style. I'll be sure to submit feedback, which is sent to our Product Development Team when considering features to introduce in software updates. QuickBooks is constantly improving thanks to the ideas provided by users, like yourself.

To know more about how to customize email templates and use them to send messages from QuickBooks, read this article: Create custom email templates in QuickBooks Desktop.

You can always get back to me if you have any other questions about QuickBooks, I'm here to answer it for you. Have a great day!